There are many types of polyethylene made and sold today. One type, in particular, is made by various suppliers and sold in large quantities. This polyethylene is called high pressure, free radical polyethylene (usually called LDPE), and is usually made using a tubular reactor, or an autoclave reactor, or sometimes a combination. Sometimes polymer users blend LDPE with other polymers, such as linear low density polyethylene (LLDPE), to try to modify properties, such as flowability, processability or density. However, there is a need for new LDPE polymers, which can have improved film optics, while maintaining other performance attributes.
Low density polyethylene resins with higher densities (greater than or equal to (≧) 926 kg/m3) are produced at reduced polymerization temperature and elevated pressure, in order to reduce the short chain branching frequency, and consequently to increase product density. Table A shows the kinetic data on the involved reaction steps, as derived by S. Goto et al; Journal of Applied Polymer Science: Applied Polymer Symposium, 36, 21-40, 1981 (Title: Computer model for commercial high pressure polyethylene reactor based on elementary reaction rates obtained experimentally) (Ref. No. 1). The temperature dependence is given by the activation energy. The higher the activation energy the more a certain reaction step will be promoted by higher temperature or reduced by lower temperatures. For polymer properties the ratio between the rate of a certain reaction step and the propagation rate is of importance.
TABLE ARate Constants of Elementary Reaction Ratesas Determined by Goto et al (Ref. No. 1)ReactionFrequencyActivation energy,Activation volume,stepfactorcal/molecm3/molePropagation5.63E+1110,520−19.7SCB5.63E12 13,030−23.5The property temperature dependence is expressed by the A Activation energy, so for SCB frequency in product: Δ Activation energy=13.03−10.52=2.51 kcal/mole.
When producing high pressure, medium density resins, low temperature conditions should be applied, in order to reduce short chain branching. With the kinetics of Goto et al., it has been found that a “930 kg/m3 density” can be produced by maintaining the average polymerization temperature at 205° C., for a reactor system operating at 2400 bar. This low average temperature can be achieved by lowering the control temperature in each zone of an autoclave process, and by lowering the control peak temperatures in a tubular reactor. The low control/peak temperatures needed for producing medium resins in a tubular reactor, result in significantly reduced heat transfer capability, and consequently, in significantly reduced conversion. For example, an average polymerization temperature of 175° C. for the first polymer fraction of a tubular reactor, where the polymerization is started at 150° C., leads to a sensible heat content of 50° C., needed to dissipate the heat of polymerization, and which, in turn, is equivalent to a conversion of only 4%.
The conversion of an autoclave tube reactor can be maintained at a much higher level, since this system depends to a larger extent on sensible heat content, which allows for a larger part of the polymer to be made at very low temperature conditions. However, an average polymerization temperature of 175° C. for the first polymer fraction of an autoclave reactor leads to a sensible heat content of 135° C., which is equivalent at adiabatic conditions to a conversion of only 10%.
Despite the significant lower conversion level, tubular medium density reactor products are still preferred in many blown film applications for the better optical film properties, due to a narrower molecular weight distribution. The molecular weight distribution of autoclave and autoclave/tube products is broadened by the increased long chain branching (LCB) frequency (due to the higher conversion and polymer concentration level), and the wide residence time distribution in the overall reactor system. In an autoclave system, some polymer molecules will stay, and continue to grow very long through the long chain branching mechanism, while other polymer molecules will stay and grow very short. The overall impact of the wide residence time distribution is broadening of the molecular weight distribution.
As discussed above, there is a need for narrow MWD polyethylene products, with improved optics, and which can be prepared at high conversion levels in autoclave or autoclave/tube reactor systems. These needs and others have been met by the following invention.